Recovery of heat treating salts



Patented July 1, 1941 2,247,579 RECOVERY OF HEAT TREATING SALTS Donald A. E. I. du mington,

No Drawing.

Holt, Niagara Falls, N. Pont de Nemours & Del., a corporation of Delaware Application April 10, 1940,

Serial No.

This invention relates to the heat treatment of metal articles and, more particularly, to a method of heat treating in fused salt baths.

A common method of heat treating metal articles consists in immersing them in a bath of fused salts. When the article has been heated to the desired temperature in the salt bath, it is removed and cooled either rapidly or slowly, be obtained. The article removed from the bath has thereon an after the metal article has been cooled, this salt film must be removed, for example, by washing with water. The salt solution resulting from washing the salt film from the treated article often is wasted, since it ordinarily is not economical to evaporate the heat treating salt. recover salt may be salt is sufllciently costly evaporation, especially depending upon the effect to adherent film of fused salt and the solution to recover At times evaporation to resorted to, provided the to Justify the cost of when a large volume of work is involved.

An object of the present invention is to facilitate removal of salt film from articles which have been heat treated in a fused salt bath. Other objects will be apparent from the following description of my invention.

I have found that when a metal article is a temperature equal to or above the melting point of the salt bath composition, the surface of the metal becomes thor- This causes formation of a salt film which adheres tightly to the metal article when it is cooled, thus forming a solid salt film which is difilcult to remove.

Generally the only feasible method of completely removing such film without damage to the heat treated article is by thoroughly washing and Commonly in fused salt a salt bath is selected which has a melting point below the desired temperature of heat treatment. The temperature of the bath then is maintained at or slightly above the detreating temperature. This results in the formation of the adherent salt film on the heated in a salt bath at oughly wetted by the fused salt.

scrubbing in water.

heat treating,

sired heat heat treated article, as described above.

I have discovered that it is possible to efiiciently bath in such manner that an adherent salt film is not To accomplish this which has a melting treatment temperature and maintain the bath at a temperature equal to, or preferably above, its melting point.

an article at a temmay utilize a bath 55 heat treat a metal article in a. fused salt left on the metalarticle.

result, I select a fused bath point above the desired heat For example, to heat treat perature of 400-500 0.. I

Y., assignor to Company, Wil- 328,875 7 Claims. (01. 14s 15) which melts at about 600 C. and maintain-the metal article to this bath for a short moved from the bath period of time and is rewhen within the desired heat treating range which is below the melting point of the bath composition. Preferably the melting point of the bath and the desired heat treating temperature are so related that the temperature of the surface of the article being heat treated is about to 200 below the melting point of the bath'comp'osition or lower when the article is removed from the .bath.

I have found that under these conditions when the metal article is introduced into the bath a substantially non-adherent film of solid salt forms as a sort of sheath around the metal article. Apparently there is suflicient air adsorbed on the surface of the metal article'to prevent close adherence of the solid salt film to the surface of the article. Since the metal article is removed from the bath before the metallic surface has reached the melting point of the bath composition, this sheath or film of solid salt is not melted. Hence, when the metal article is removed from the bath, it carries with it the substantially non-adherent sheath of solid salt which has a film of the fused salt clinging to the outside. When the article is' removed from the bath, the fused salt on the exterior of the solid film of salt is rapidly cooled by the air so that a sheath of solid salt is formed. This loosely adherent sheath of solid salt then may be practically completely removed from the metal article by any mild mechanical treatment such as tapping, fieXing, vibrating, scraping or the like, which causes the sheath to break and drop off. In this manner, substantially all of the salt removed from the bath with the metal article is instantly recovered in a. dry formand without further treatment can be returned to the bath or stored for further use.

In one method of practicing my invention, I first select a fused salt bath composition which has a. melting point above the desired heat treatment temperature and maintain this bath at a constant temperature within desired limits, at or above its melting point. A series of the metal articles to be heat treated are then immersed in the bath for varying periods of time and each sample is tested by suitable known means to determine whether it has received the proper heat treatment. In this way the required time of heat treatment in the bath is determined;

the surface of the article in the bath has reached a temperature 1 thereafter the metal articles are heated in the bath for the period of time thus established.

The time required for heat treating a metal article in the bath in accordance with my invention will vary depending upon the speciflc heats a heat treating temperature. My process is especially well adapted for heat treating articles of relatively small thickness, made of metal which has relatively low thermal conductivity, for example, steel sheet, steel wire and the like or articles fabricated therefrom. For example, I may pass steel wire, strip or sheet through the salt bath in a continuous manner and thence pass the steel through flexing rolls or other device adapted to flex wire or strip at a point where the material has cooled sufficiently to completely solidify the loosely adhering salt. The flexing of the steel causes substantially all of the salt film to drop off. If desired, the flexing device may be located above the bath so that the solid salt which drops off is returned by gravity directly to the bath. Thereby,'the drag out" loss is practically nil.

I have found that the loosely adhering salt film adequately protects the metal from oxidation prior to cooling. Hence, it is generally preferable to cool the hot metal to a low temperature at which rapid oxidation does not occur before application of flexing or other mechanical treatment to remove the salt fllm.

While I generally prefer to cool the heat treated.

article in the air, my invention is not so restricted. For example, the metal may be quenched in a bath of oil or other liquid which does not dissolve the salt fllm. Alternatively, I may first-remove the salt fllm as described above and then quench in water or aqueous salt solution to further cool .the metal to a low temperature.

The various known fused salt baths may be used to practice my invention. These may consist of single salts or mixtures of salts, for example chlorides and other halides, carbonates, nitrates and nitrltes of the alkali and alkaline earth metals. The baths also may contain suitable known cementation or case hardening ingredients, e. g. cyanides, cyanamides, hydrocyanic acid polymer and the like, when desired.

.1 claim:

1. A process for heat treating a metal article to a predetermined temperature which comprises immersing said article in a salt bath, the melting point of which is higher than said predetermined temperature, the temperature of said bath being higher than said melting point, removing said article from said bath when the temperature of said article has risen to said predetermined tempoint of which is higher than said predetermined thereof.

temperature, the temperature of said bath being higher than said melting point, article from said bath when the said article has risen to said perature, cooling said article and flexing said article to temperature of 3. A process for heat treating a steel article 'to a predetermined temperature which comprises immersing said article in a salt bath, the melting point of which is higher than said predetermined temperature, the temperature of said bath being higher than said melting point, removing said article from said bath when the temperatureof said article has risen to said predetermined temperature, cooling said article and removing loosely adhering solid 'salt from the surface thereof.

4. A process for heat treating a steel sheet to a predetermined temperature which comprises immersing said sheet in a'salt bath, the melting point of which is higher than said predetermined temperature, the temperature of said bath being higherthan said melting point, removing said sheet from said bath when the temperature of said article has risen to said predetermined temperature, cooling said sheet and flexing the sheet so as to remove solid salt from the surface thereof. V

5. A process for heat treating a metal article which comprises immersing. said article in a said bath before the temperature of the surface of the article has reached the melting point of said bath, cooling said article in the air and removing loosely adhering solid salt from the cooled article.

7. A process for the heat treating of a steel sheet which comprises immersing said sheet in a fused salt bath, withdrawing said sheet from said bath before the temperature of the surface of the article has reached the melting point of said bath, cooling said sheet in the air and flexing the cooled sheet so as to remove solid salt from the surface thereof.

DONALD A. HOLT.

removing said predetermined temloosen solid salt from the surface 

